Fundamentals of Solar Radiation

The definition of time and different time systemsReminder – geocentric and geographic coordinates – angle of incidenceOrbit of the Earth around the sun – distance between the sun and the EarthSolar declinationDefinitions of time: from year to secondMean solar time – true solar timeCoordinated universal time and legal timeCoordinated universal time (UTC)Legal time – civil time – time zoneTo sum up: converting the legal time to the true solar timeThe course of the sun over an observer on the groundPosition of the sun seen by an observer on the groundSolar zenithal angle – solar elevation – azimuthDirection of the sun in the case of an inclined planeSolar zenithal angles at solar noon during a yearTimes of sunrise and sunsetDaytime – daylengthSolar angles during equinoxes and solsticesEffective solar anglesMethods for computing effective anglesHow to practically use one of these methodsSolar radiation incident at the top of the atmosphereRadiance – irradiance – irradiationRadianceIrradianceIrradiation – radiant exposureConverting irradiance to irradiation and vice versaSolar activity – extraterrestrial radiationExtraterrestrial radiation received on a horizontal planeTypical values of hourly extraterrestrial total radiationDaily extraterrestrial radiationExtraterrestrial radiation received on an inclined planeSpectral distribution of the extraterrestrial radiationRadiative transfer in the atmosphereAbsorption by gases in the atmosphereScattering – overview – case of the air moleculesAttenuation – extinctionQuantities related to the attenuationAir massOptical depth – optical thicknessTransmittanceLinke turbidity factorVisibilityEffects of aerosols on radiationEffects of clouds on radiationThe paths of the sun rays in the atmosphereSummary of contributions of atmospheric constituents to attenuation of radiationGround reflectionReflection factor – its spectral variationsReflection factorExamples of spectra of reflection factorBidirectional reflectance distribution function (BRDF)AlbedoSolar radiation received at ground levelComponents of the solar radiation at ground levelDirect, diffuse, and reflected components – global radiationSeveral definitions of the direct componentDiffuse fraction – direct fractionClearness indexDefinitionAn example of 15-min clearness indicesAn example of monthly clearness indicesClear-sky indexThe prominent role of cloudsAn example of hourly radiation over the North Atlantic OceanMap of a multi-year average of the total solar irradiance at groundSunshine durationThe most important variables for the solar radiation at groundDecoupling the effects of clear atmosphere and cloudsSpectral distribution of the solar radiation at groundSpectral distribution of the irradiance in a cloud-free atmosphereInfluence of the turbiditySpectral distribution of the direct and diffuse componentsInfluence of the solar zenithal angleTypical irradiances integrated over some spectral intervals in a clear atmosphereSpectral distribution of the irradiance in a cloudy atmosphereInfluence of the cloud optical depthInfluence of the solar zenithal angleTypical irradiances integrated over spectral intervals in cloudy conditionsVariability – implications for estimating radiationTime and space scalesRandom nature of meteorological variables – ergodicityFluctuations around the average – small-scale variabilityMeasurement – samplingSampling theorem – detection of the smallest detailsSampling theoremWhat are the smallest observable details in a series of measurements?Ground-based instruments for measuring solar radiation at groundCommon ground-based instrumentsThermopiles and photodiodes – calibration and classes of instrumentsPyranometer – pyrheliometer – pyranometer with a shadow ring or ballOther instruments measuring global radiation and its diffuse component simultaneouslyTypical uncertainties of pyranometers and pyrheliometersIncomplete coverage in both space and time by stationsCalculation of daily, monthly, and yearly sums and averagesIdeal case of a complete time series of measurementsCalculation of climate normals in meteorologyCase of incomplete time seriesOverview of the completion of a time seriesOther means for estimating solar radiation at surfaceMeteorological analyses and reanalysesImages acquired by Earth observing satellitesMultispectral imageryOrbits of satellitesOverview of the methods for estimating the solar radiationEstimate using nearby ground stationsUsual approach to estimate components from the only knowledge of global radiationMethods using other meteorological variablesÅngström’s relationship between irradiation and sunshine durationRelationships between the total radiation and that in a given spectral rangeEstimate using other meteorological variablesControl of the plausibility of measurementsObjective of the control of plausibilityHow to control the plausibility of measurementsChecking metadata: an overviewChecking metadata: geographic location of the stationChecking metadata: time system and time stampOverview of the most common problemsSome graphs for the visual inspection of the time-related problemsBenefit in using the hourly extraterrestrial irradiationChecking metadata: checking the measured quantity and its unitVisual and automated proceduresThe principle of visual inspectionVisual inspection of daily measurementsDaily and monthly total irradiationAn example of visual control of daily spectral valuesVisual inspection of hourly and intra-hourly valuesTwo-dimensional representation of hourly and intra-hourly valuesAutomatic proceduresHourly and intra-hourly measurementsDaily measurementsMonthly measurementsSpectral radiation measurements
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